Fructose intolerance: description
Fructose intolerance is a form of food intolerance. Affected persons tolerate fructose only to a limited extent or not at all. There are two forms of the metabolic disorder – fructose malabsorption and hereditary fructose intolerance:
Different forms of fructose intolerance.
Fructose malabsorption
According to the Allergy Information Service, fructose malabsorption is relatively common: In Europe and North America, it affects about one in three adults and two in three young children.
Fructose malabsorption can disappear in the course of life. Or it can be managed with an adapted diet (low fructose depending on individual tolerance).
Hereditary fructose intolerance (HFI)
This potentially life-threatening defect in fructose metabolism already occurs in infancy. Even small amounts of fructose can cause severe health complications (e.g. kidney and liver damage) in affected individuals.
Hereditary fructose intolerance persists throughout life and requires a special diet throughout life.
Fructose intolerance: symptoms
Hereditary fructose intolerance has more serious consequences: Here, the consumption of fructose can cause confusion, dizziness, sweating and even seizures and coma, in addition to nausea and vomiting. Liver and kidney damage are also possible.
Read more about possible symptoms of fructose intolerance in the article Fructose intolerance – symptoms.
Fructose intolerance: causes and risk factors
If one exceeds the natural absorption capacity for fructose, the excess fructose enters the large intestine. There it is broken down by bacteria, producing gases (including hydrogen, carbon dioxide and methane) and short-chain fatty acids. These can lead to complaints such as flatulence, diarrhea or constipation.
Intestinal fructose intolerance (fructose malabsorption) is only present when someone develops unpleasant symptoms such as flatulence or diarrhea after ingesting just 25 grams of fructose (or even less). People with hereditary fructose intolerance cannot tolerate any fructose at all.
Fructose malabsorption – disturbed sugar transport
In fructose malabsorption, there is a defect in this transporter. This restricts the absorption of fructose from the intestine. As a result, even a relatively small amount of fructose can no longer be handled and continues to enter the large intestine.
The transporter disorder can be temporary (e.g., in acute gastrointestinal inflammation) or permanent or congenital (e.g., in people with Crohn’s disease).
Fructose is found not only as a single sugar in fruits, but also in ordinary household sugar (sucrose): This is a dual sugar consisting of fructose and glucose (dextrose).
Physical training, on the other hand, is said to worsen the transport capacity of GLUT 5. Sorbitol, a sugar alcohol that is often added to foods as a sugar substitute or humectant (E420), also has an unfavorable effect. It uses the same transporter as fructose for its passage through the intestinal wall and can therefore compete with it.
Hereditary fructose intolerance – congenital enzyme deficiency
One of the degradation steps requires the enzyme fructose-1-phosphate aldolase. If it is not present in sufficient quantities, an intermediate product of fructose degradation accumulates (fructose-1-phosphate). This inhibits enzymes that the body needs to burn its most important energy source – glucose – for energy production (glycolysis) or to produce new glucose when energy requirements increase (gluconeogenesis).
Fructose intolerance: examinations and diagnosis
The right person to contact if you suspect fructose intolerance is an internal medicine specialist who specializes in diseases of the digestive tract (gastroenterologist).
First, the doctor will take your medical history by asking you about your current complaints and any previous illnesses (medical history). Possible questions here include:
- Do you often suffer from flatulence, abdominal pain or bloating lately?
- Have you discovered a connection with certain foods?
- Do the symptoms improve when you avoid these foods?
- Does a related family member have fructose intolerance?
- Do you suffer from lactose intolerance?
Since the intestine can only be assessed from the outside to a limited extent, further examinations are usually necessary for a fructose intolerance diagnosis. The fructose intolerance test, in which the hydrogen content in the breath is measured, is a central component of the diagnosis. In addition, a blood sample may be taken to detect hereditary fructose intolerance and rule out other diseases.
Fructose intolerance test
What does the test result indicate? An increased concentration of hydrogen in the exhaled air indicates fructose malabsorption. This is because when bacteria in the large intestine break down the fructose because the transport via the small intestine does not work or works only to a limited extent, hydrogen (H2) is produced in the process. This migrates into the lungs and from there into the exhaled air.
Hereditary fructose intolerance test
If symptoms of fructose intolerance occur shortly after birth or if close relatives suffer from hereditary fructose intolerance, the physician can perform a genetic test based on a blood analysis: Characteristic gene changes provide evidence of hereditary fructose intolerance.
By analyzing tissue samples from the liver, kidney or small intestine, the genetically determined enzyme deficiency can be detected.
Fructose intolerance: treatment
In hereditary fructose intolerance, foods with fructose are completely taboo. This applies not only to solid food, but also to beverages. For example, with hereditary fructose intolerance, alcohol and soft drinks must be eliminated from the menu, as they often contain fructose.
Instead, people with fructose malabsorption are recommended a multistep nutrition therapy in collaboration with a dietitian.
- First, the consumption of fructose-containing foods is restricted for a limited time (about two weeks).
- Once the individual fructose tolerance has been determined, permanent and individual dietary recommendations can be developed and implemented together with the nutritionist. The goal is a wholesome diet that provides all the necessary nutrients in sufficient quantities, taking into account individual preferences, habits and tolerances as far as possible.
Fructose intolerance – table
Fructose intolerance: disease course and prognosis
Hereditary fructose intolerance persists throughout life. To avoid symptoms and serious damage (such as liver and kidney damage), affected individuals must always and permanently follow a fructose-free diet. Sorbitol and sucrose (household sugar) are also prohibited.
